The present invention generally relates to information signal recording mediums such as video discs and digital audio discs, and more particularly to an electrostatic capacitance type high density information signal recording medium which is recorded on a surface thereof with information signals as variations in geometrical configuration. The recorded information signals are reproduced from the information signal recording medium as variations in electrostatic capacitance formed between the information signal recording medium and an electrode of a reproducing stylus which relatively scans over the surface.
Recently, a system has been reduced to practice and has become popular wherein a reproducing apparatus reproduces recorded information signals from an electrostatic capacitance type information signal recording disc. The information signals are recorded on the disc as variations in geometrical configuration such as formation of pits. The reproducing apparatus reproduces the recorded information signals from the disc as variations in electrostatic capacitance between the disc and an electrode of a reproducing stylus. This type of disc is used as a video disc which is recorded with video and audio signals, an audio disc which is recorded with audio signals, and the like. In a certain kind of the audio disc, the disc is recorded with video signals related to still pictures in addition to the audio signals.
In any kind of electrostatic capacitance type information signal recording disc, a part of the disc containing the variations in geometrical configuration functions as an electrode which forms the electrostatic capacitance between the electrode of the reproducing stylus. Thus, this part of the disc is made of a conductive material. The conventional electrostatic capacitance type information signal recording disc is formed by heating and pressing, by use of a die provided with stampers, a conductive resin composition which is obtained by admixing a conductive material such as carbon black powder into polyvinyl chloride resing.
However, according to the conventional electrostatic capacitance type information signal recording disc, the entire disc is formed from the conductive resin. Hence, even the central part of the disc in the thickness direction thereof which does not contribute to the signal reproduction, is formed from the conductive resin. But as is well known, the carbon black powder is relatively expensive, and the conductive resin is accordingly expensive. Therefore, the conventional disc which is formed from the conductive resin in its entirety is expensive, and there is a disadvantage in that no measures can be taken to reduce the manufacturing cost of the disc because the conductive resin is expensive.
When manufacturing the conventional disc, a tablet which is made of a conductive resin and has a size such that the diameter thereof is approximately 1/3 the diameter of the disc is prepared. This tablet is heated and pressed by a die provided with a pair of stampers and is then stamped, so as to obtain a disc having a predetermined thickness and having the variations in geometrical configurations formed on the surface thereof. However, when the tablet (conductive resin) is heated and pressed by the stampers and stamped, the conductive resin is spread between the stampers. Hence, the stamping surfaces of the stampers are easily damaged by the spreading conductive resin which rubs the stamping surfaces. As a result, there is a disadvantage in that the serviceable life of the stampers is short.
In addition, it takes a certain time to prepare the tablet, and there is a disadvantage in that it is difficult to reduce the disc manufacturing cycle. Furthermore, in order to prepare uniform tablets, it is necessary to adjust a tablet forming machine which is provided with respect to each disc pressing (compresion molding) machine which forms the disc. Consequently, there is a disadvantage in that a troublesome operation is required to adjust the tablet forming machine.
On the other hand, a disc forming method in which the disadvantage described before that the disc is expensive because the entire disc is made from the conductive resin containing the expensive carbon black powder is eliminated, was proposed in a Japanese Laid-Open Patent Application No. 58-55224 (Japanese Patent Application No. 57-154470 filed Sept. 3, 1982). This previously proposed disc forming method comprises the steps of forming sheets which are made from a conductive plastic containing the carbon black, positioning the sheets with respect to the stampers within a metal die which is used for compression molding, placing a core material which is made from a non-conductive plastic and is in the form of a dumpling so that the core material may be subjected to the compression molding between the sheets, and forming a disc having such a sandwich type construction that the surface parts of the disc are conductive and the central part of the disc is non-conductive.
According to the disc forming method proposed in the above Japanese Laid-Open Application, the quantity of conductive plastic which is required can be reduced by a quantity corresponding to the core material. Thus, the quantity of conductive plastic and accordingly the quantity of the expensive carbon black which is required to form one disc can be reduced, and there is an advantage in that the manufacturing cost of the disc can be reduced.
However, according to the disc forming method proposed in the above Japanese Laid-Open Application, bubbles are easily mixed between the sheet and the core material when the stampers subject the sheets and the core material interposed therebetween to the compression molding so as to obtain the disc. Further, dust particles or the like easily adheres to the core material when supplying the core material between the sheets, and the dust particles or the like are also subjected to the compression molding. In this case, there is a disadvantage in that the bubbles and foreign substances such as the dust particles remain between the sheet and the core material of the disc which is formed by the compression molding. Such a disc contains irregularities on the signal recording surface thereof, and the recorded signals cannot be reproduced in a normal manner at parts of the recording surface containing the irregularities. In addition, when forming the disc by the compression molding, the sheets and the core material must be supplied to the compression molding machine independently. However, it is difficult to supply the sheets and the core material independently in a satisfactory manner, and the construction of an apparatus for independently supplying the sheets and the core material to the compression molding machine is complex. Moreover, it takes a certain time to complete the pressing cycle, and there is a disadvantage in that virtually no measure can be taken to reduce the pressing cycle. Further, when the stampers form the disc by the compression molding, the stampers must press and spread the core material indirectly, that is, through the sheets. For this reason, much of the pressing force of the stampers is used up to spread the core material, and there is a disadvantage in that the variations of geometrical configurations such as the formation of pits cannot be formed on the recording surface of the sheet in a satisfactory manner.
Accordingly, in order to overcome the disadvantages of the conventional method described heretofore, an electrostatic capacitance type information signal recording medium and manufacturing method thereof were proposed in a U.S. patent application Ser. No. 686,156 filed Dec. 26, 1984 in which the assignee is the same as the assignee of the present application. The novel electrostatic capacitance type information signal recording medium proposed in the U.S. Patent application Ser. No. 686,156 is formed by preparing a plate member having such a laminated structure that a conductive resin sheet which is used as the surface material is provided on one or both surfaces of a core sheet and stamping on the surface of the conductive resin sheet part of the plate member variations in geometrical configurations in accordance with signals to be recorded on the disc.
However, in the case of the recording medium proposed in the Japanese Laid-Open Application or the U.S. Application, it has been found that the variations in the geometrical configuration such as pits may not be formed satisfactorily on the conductive surface material resin sheet when the recording medium is compression molded or that the conductive surface material resin sheet may be cut. When such unsatisfactory formation of the variations in geometrical configuration or the cut in the conductive surface material resin sheet takes place, a large number of defective products are produced during the compression molding and the yield of the manufactured recording mediums becomes poor.
Accordingly, the present inventors continued research with respect to the poor yield when the recording mediums are compression molded. As a result of the research, the present inventors have found that the fraction defective of the compression molded recording mediums greatly changes depending on the relationship of the magnitudes of the melt viscosities of the non-conductive core material and the conductive surface material, and have completed the present invention based on the findings.
In other words, at the temperature (for example, 180.degree. to 190.degree.) which is reached when the compression molding takes place in the compression molding machine, the core material melts and spreads before the surface material due to the pressing pressure at the time of the compression molding when the melt viscosity of the core material is within a certain range close to the melt viscosity of the surface material, and the pits cannot be formed on the surface material satisfactorily. Hence, it is desirable that the melt viscosity of the core material is greater than the melt viscosity of the surface material. However, as a result of research, it has been found that the melt viscosity of the core material may be slightly smaller than the melt viscosity of the surface material, because although the temperature of the stamper of the compression molding machine is immediately transmitted to the surface material at the time of the compression molding, there is a time delay before the temperature is transmitted to the entire core material.